Epoxy resins have been widely used for many years in making solid products, including molding compositions and fiber reinforced structural plastics, and also casting or potting compositions, as well as coatings and adhesives. Epoxy resins are particularly advantageous because they are easily converted from liquid or pasty or thermoplastic initial materials into strong and chemically resistant thermoset products, with minimum shrinkage and without evolution of volatile materials. Nevertheless, such compositions tend to be brittle when they are sufficiently crosslinked to exhibit a desirably high heat distortion temperature, and previous attempts to diminish the brittleness and produce a high impact resistance by the usual means, such as introduction of plasticizers or flexibilizers, have not been completely satisfactory because other desirable properties, such as strength, have been sacrificed. Accordingly, an object of this invention is production of thermoset epoxy resin compositions which are not brittle but have a high resistance to impact and crack propagation without reduction of strength.